PTC thermistors are well known to the art. Such devices have previously been used as heaters, and are generally made from doped barium titinate ceramics that have been coated with metallized electrodes. When the electrodes are connected to a current supply, the doped barium titinate ceramic exhibits a sharp positive temperature coefficient of resistance. The ceramics are designed so that below a critical temperature, their anomoly temperature, the resistance of the material remains at a low value and is essentially constant. When the predetermined temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystalline structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the thermistor's resistance of several orders of magnitude over a small temperature change. For example, barium titinate thermistors with a room temperature resistance of 3.0 ohms can increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the crystalline phase change takes place can be adjusted in the manufacturing process and can be established between about -60.degree. F. and 600.degree. F., and even higher. When energized with a suitable current by applying voltage to the opposite sides of the PTC thermistor, the ceramic rapidly heats up to a predetermined operating temperature and then "locks in" at this temperature. This rapid heating is due to the initial low resistance of the PTC thermistor which results in an initial high power of the heater. The "lock in" is due to the abrupt increase in resistance which causes the generated power to be reduced until it equals the dissipated power. At this point, thermal equilibrium is achieved and the PTC thermistor becomes self-regulating at the predetermined temperature.
Commonly, PTC thermistors that are used for heaters are attached to a heat sink and are electrically isolated from the media being heated through an electrical isolation means. The temperature of the media to be heated is dependent upon the heat flow through the PTC, the heat sink, and the electrical isolation means. As the temperature of the PTC thermistor is increased, the temperature drop between the PTC thermistor and the media being heated is significantly increased. When the heat flow decreases, as in the case of a water heater or an evaporator that becomes void of water, the heater temperature increases and can easily exceed safe limits of about 300.degree. F. In addition, from time to time, the PTC thermistors may individually vary in their anomoly temperatures which can cause significant changes in heater performance from heater to heater. Moreover, the construction of the heater can vary from unit to unit. thereby also causing changes in heater performance.
We have found that the disposition of a PTC thermistor in a sealed, thermally conductive cannister which is filled with an electrically inert fluid which boils at or below the anomoly temperature of the PTC thermistor and condenses at a temperature below the boiling point of the media being heated can provide an excellent mechanism for heating a wide variety of medias, especially liquids. The inert fluid is boiled in the cannister by the PTC thermistor, and the vapor condenses upon the inner walls of the cannister because the condensation temperature is less then the temperature of the media being heated. When the vapor condenses, it gives up its latent heat of vaporization and heats the cannister which will radiate the heat to the media being heated. The cannister cannot be heated above the boiling point of the fluid held within it. The heater of the present invention provides an efficient heat transfer from the PTC thermistor through the use of the phase change of the dielectric fluid from liquid to vapor and the subsequent phase change of condensing of the vapor on the walls of the cannister. Safe operating limits cannot be exceeded because the PTC thermistor cannot exceed its anomoly temperature and the fluid cannot be heated above its boiling point.
Preferably, the fluid that is used is one of the Fluorinert family of electronic fluids. Fluorinert is a registered trademark of the 3M Corporation. Fluorinert fluids are a family of completely fluorinated organic compounds which are formed from common organic compounds by replacement of all carbon bound hydrogen atoms with fluorine atoms. These fluids are extremely non-polar and have low solvent action. They are colorless, odorless, low in toxicity and non-flamable. Because of a high thermal stability and low chemical reactivity, the fluids leave essentially no residue upon being heated or boiled. The fluids further provide a high degree of electrical protection, that is electrical insulation, and can have boiling points well within the necessary ranges for the heater of the present invention. Conveniently, a desired boiling point can be selected from a variety of boiling points available from the family of Fluorinert fluids. Typically, these boiling points can be between about 50.degree. and 250.degree. C. As stated previously, in the preferred embodiment of this invention, the anomaly temperature of the PTC thermistor is equal to or above the boiling point of the fluid that is used and is also selected to be one which is below a predetermined maximum temperature of the heater.